1. Field of the Invention
The present invention relates to desalination, and particularly to a system for producing desalinated water from saltwater, such as seawater using both multi-effect distillation and multi-stage flash evaporation.
2. Description of the Related Art
A falling film evaporator is an industrial device to concentrate solutions, especially with heat sensitive components. The evaporator is a special type of heat exchanger. In general, the evaporation takes place on the outside surfaces of horizontal or vertical tubes, although it should be noted that there are also applications where the process fluid evaporates inside vertical tubes. In all cases, the process fluid to be evaporated flows downwards by gravity as a continuous film. The fluid creates a film along the tube walls, progressing downwards, hence the name “falling film”.
In a falling film evaporator, the fluid distributor must be designed carefully in order to maintain an even liquid distribution for all tubes along which the solution falls. In the majority of applications, the heating medium is placed inside the tubes, thus high heat transfer coefficients can be achieved. In order to satisfy this requirement, condensing steam is commonly used as a heating medium.
For internally evaporating fluids, separation between the liquid phase (i.e., the solution) and the gaseous phase takes place inside the tubes. In order to maintain conservation of mass as this process proceeds, the downward vapor velocity increases, increasing the shear force acting on the liquid film and therefore also the velocity of the solution. The result can be a high film velocity of a progressively thinner film, resulting in increasingly turbulent flow. The combination of these effects allows very high heat transfer coefficients.
The heat transfer coefficient on the evaporating side of the tube is mostly determined by the hydrodynamic flow conditions of the film. For low mass flows or high viscosities, the film flow can be laminar, in which case heat transfer is controlled purely by conduction through the film. Therefore, in this condition, the heat transfer coefficient decreases with increased mass flow. With increased mass flow, the film becomes wavy laminar and then turbulent. Under turbulent conditions, the heat transfer coefficient increases with increased flow. Evaporation takes place at very low mean temperature differences between heating medium (i.e., process stream) and film liquid, typically between 3 K and 6 K, thus such devices are ideal for heat recovery in multi effect processes.
A further advantage of the falling film evaporator is the very short residence time of the liquid and the absence of superheating of the same. The residence time inside the tubes is typically measured in seconds, making it ideal for heat-sensitive products such as milk, fruit juice, pharmaceuticals, and many other mass-produced liquid products. Falling film evaporators are also characterized by very low pressure drops, thus they are often used in deep vacuum applications as well.
However, due to the intimate contact of the film liquid with the heating surface, such evaporators are susceptible to fouling from precipitating solids; liquid velocity, typically low at the top rows of a bank of horizontal tubes, is usually not sufficient to perform an effective self-cleaning of the tubes. Falling film evaporators are therefore typically used only with clean, non-precipitating liquids.
Falling film evaporation is the primary principle used in multi-effect distillation (MED) systems (sometimes also referred to as “multiple-effect distillation systems”). Multi-effect distillation is a distillation process often used for sea water desalination. It consists of multiple stages or “effects”. In each effect, the seawater feed falls as a film over the outside surfaces of the tubes and is heated by steam inside the tubes. Some of the falling water film evaporates, and this vapor flows into the tubes of the next effect, heating and evaporating more water. Each effect essentially reuses the energy from the previous effect. Although the tubes can be submerged in the feed water, it is far more common that the seawater feed is sprayed on the top of a bank of horizontal tubes, and then drips from tube to tube until it is collected at the bottom of the effect.
FIG. 2 illustrates a typical prior art multi-effect distillation evaporator 100. In the first effect 102, seawater is fed, via an inlet 108, to one or more sprayers or nozzles 110 positioned within the first effect 102. Heated steam, produced by an external boiler or the like, is fed through a tube 112. As the sprayed seawater lands on the external surface of the tube 112, and forms a thin liquid film thereon, heat transferred from the heating steam causes the seawater to evaporate, forming water vapor V. The heat transfer cools the steam, producing condensate in the tube 112, which is then returned back to the boiler for subsequent re-heating. The seawater which does not evaporate (indicated as S in FIG. 2), drips from one portion of the tube 112 to another (or from tube to tube, in the case where multiple such tubes are used), until it is collected at the bottom 114 of the first effect. A pump 116 then delivers this collected seawater into the second effect 104, where it is sprayed by sprayers or nozzles 120, similar to the spraying in the first effect 102.
The water vapor V from the first effect is transferred by a second tube 118 into the second effect and acts in a similar manner to the steam passing through tube 112 in the first effect, except that the condensate in second tube 118, rather than being returned to the boiler, is drawn out through a product conduit 124, where distilled water is collected. The seawater S which does not evaporate into water vapor V in the second effect 104, once again, falls from tube portion to tube portion (or tube to tube) to be collected on the bottom 122 of the second effect 104. A pump 126 then delivers this collected seawater into the third effect 106, where it is sprayed by sprayers or nozzles 128, similar to the spraying in the first and second effects 102, 104.
The water vapor V from the second effect 104 is transferred by a third tube 130 into the third effect 106 and acts in a similar manner to the steam passing through tube 112 in the first effect 102 and the heated vapor passing through the second tube 118 in second effect 104. In the third effect 106, the condensate in third tube 130 is drawn out through the product conduit 124, where it mixes with the desalinated water from the second effect 104 to be collected. The seawater S which does not evaporate into water vapor V in the third effect 106, once again, falls from tube portion to tube portion (or tube to tube) to be collected on the bottom 132 of the third effect 106, where it is then pumped, by pump 134, to the next effect. Although only three effects 102, 104, 106 are shown in FIG. 2, it should be understood that this is shown for exemplary and illustrative purposes only. An example of a conventional multi-effect distillation system is shown in U.S. Pat. No. 3,481,835, which is hereby incorporated by reference in its entirety.
Conventional multi-effect distillation systems, such as the above, which generally rely on falling film evaporation, suffer from a number of drawbacks, each of which typically limits the design capacity of the units and the maximum permissible operating temperatures. On a broad level, many MED designs involve complex and often circuitous paths for heated seawater and vapor to minimize usage of pumps, maintain wettability of the tubes to avoid scaling, and to maximize energy recovery from the flashing brine and distillate. The farther the pumps, vessels, water routes and vapor routes are from minimal, optimized paths, the more the design suffers from excessive losses.
In addition to multi-effect distillation systems, multi-stage flash (MSF) evaporation is also relatively commonly used to produce desalinated water from saltwater sources, such as seawater. FIG. 3 shows a conventional prior art MSF system or plant 200, where feed seawater or brine enters the system under pressure, being drawn into the plant 200 via a pump 228 or the like. The seawater or brine is transported, under pressure, through conduits or pipes 232 to a brine heater 214, which then delivers heated brine to flash chambers 216. A steam generator 212, which is a separate simple steam power plant, external to the MSF system, supplies the brine heater 214 with the heating steam needed to heat up the brine. The steam generator 212 is a simple steam power plant (preferably a Rankine cycle power plant), and consists of a pump 240, a boiler 242, and a steam turbine 244, in addition to the condenser 214, which also acts as the brine heater. It should be understood that the steam turbine 244 shown in FIG. 3 is not a component of a typical MSF process, but is merely shown in this example as part of an exemplary plant utilizing MSF. The steam passing to the brine heater 214 may be extracted from a turbine, such as steam turbine 244, or may be fed directly from boiler 242. It should be understood that the simplified illustration of FIG. 3 is provided merely to describe a conventional MSF process and system. Typically, a de-superheater would also be used to condition the steam, whether extracted from a turbine or passing directly from a boiler, prior to entering the brine heater 214, thus ensuring that the steam is saturated and not superheated. Conventional MSF systems are well-known. U.S. Pat. Nos. 3,966,562 and 8,277,614, both of which are hereby incorporated by reference in their entirety, show conventional MSF systems.
As shown, the seawater or brine may also be first drawn through a cooler 230 in order to reduce the temperature of the feed, thus also the temperature of the last stage. The brine is then passed through the feed heater conduits 232. The feed heaters are condenser type heat exchangers where feed is heated by the heat released from condensing the vapor flashed off in each stage. Feed brine reaches the first stage at an elevated temperature, however it is not high enough to start flashing, and therefore, additional heat must be supplied to the brine. The brine heater 214 receives steam from the external steam generator 212, and elevates the brine temperature to the level suitable to start flashing. The brine is then injected into the flash chambers 216. It should be understood that the number of flash chambers 216 shown in FIG. 3 is shown for exemplary purposes only, and is a simplification of the number of flash stages. Typical MSF plants have between fifteen and forty stages or flash chambers. The brine delivered by the heater 214 typically has a temperature of between approximately 90° C. and 120° C., depending upon the chemical treatment or scale prevention technique used, the quality of heating steam, and the ejection system maintaining pressure in each stage.
The operating pressure in the flash chambers 216 is lower than that in the heater, thus causing the heated brine to rapidly boil or “flash” into vapor. Typically, only a small percentage of this water is converted into vapor. Consequently, the remaining water will be sent through a series of additional stages or flash chambers 216, as shown, each possessing a lower operating pressure than the previous chamber. The brine is delivered through each successive flash chamber 216 or stage through any conventional method. As vapor is generated, it is condensed in the same stage or flash chamber on the pipes 232, which run through each chamber. The condensed water is then collected by collection trays 218 and is removed by a pump 220 to produce a stream of desalinated water 222. The pipes 232 and trays 218 form the condensers for each flash stage. The remaining brine with a high saline concentration may be drawn out by a separate pump 224, and removed as waste at 226.
In the MSF process, heat transfer surfaces, which are on the brine side, are never subject to change of phase and are always kept wet and relatively free of scale precipitation by effective scale control techniques, typically involving chemical treatment of feed water and on-line mechanical cleaning. Flashing of the brine occurs at a safe distance from heat transfer tubes. This procedure makes the MSF process fairly protected from scale formation and precipitation up to the temperatures at which sulfate-based scales begin to form (i.e., above 121° C.).
In the MED process, on the other hand, evaporation takes place directly on the outside surfaces of the heat transfer tubes as the brine film reaches the liquid superheat temperatures needed for the change of phase to occur. Such an evaporation mechanism makes heat transfer surfaces highly vulnerable to scale formation and precipitation, especially since only chemical treatment can be used to retard scale formation while on-line mechanical cleaning is not possible. This situation imposes severe restrictions on the maximum practical operating temperatures in the MED process, which must be kept within a safe range (i.e., below 70° C.).
The conventional MSF process suffers from three primary sources of thermodynamic loss, namely boiling point elevation loss, pressure drop loss, and non-equilibrium loss. The boiling point elevation loss is due to the presence of salts at high concentrations in the brine, thus it is a loss that must be present in any process involving boiling or change of phase and its value depends on the state of the brine solution in terms of its temperature and concentration. Boiling point elevation loss increases with temperature as well as with concentration. In the MSF process, both driving forces of the boiling point elevation act conversely, since flashing brine temperature decreases while its concentration increases as the brine flows toward the lower temperature stages. Consequently, the resulting effect of this behavior minimizes the variations in the boiling point elevation across the MSF stages.
The pressure drop caused by the flow of vapor through the demisters and through the tube bundle results in vapor expansion, which is accompanied by a drop in its corresponding saturation temperature. This is known as pressure drop loss and it is far less in magnitude as compared with boiling point elevation or non-equilibrium losses, and it usually increases as the brine flows toward the lower temperature stages. The non-equilibrium loss, unlike the previous two losses, is an inherited characteristic of the MSF process. The amount of this loss is inversely proportional to the stage thermal level and it is directly proportional to the flashing brine depth. To illustrate such a characteristic, one can define the vapor equilibrium temperature in the brine pool at a given depth below the surface as T*b=T*0+
                              ⅆ                      T            *                                    ⅆ          P                    ·      γ        ⁢                  ⁢          h      b        ,where T*0 is the vapor equilibrium temperature at the stage pressure,
      ⅆ          T      *            ⅆ    P  represents the rate of change in vapor saturation temperature vs. pressure, and γhb represents the hydrostatic pressure in the brine pool at a given depth hb below the surface.
FIG. 4 shows plots for T*b at different values of hb over typical flashing ranges of the conventional MSF process. FIG. 4 shows that the effect of the brine depth on vapor equilibrium temperature in the brine pool is quite insignificant for high thermal level stages and it becomes rapidly significant for the lower thermal level stages. In other words, taking (Tb)in and (Tb)out as the brine bulk temperatures at the stage inlet and outlet less the boiling point elevation, then (Tb)in>T*b is a condition necessary for evaporation to occur at any point on the brine surface and below to a maximum depth of hb. For high thermal level stages, this condition is usually furnished even for the maximum submergence in the brine pool. However, for evaporation to remain effective as the brine travels through the stage towards its outlet, the condition (Tb)out>T*b must be sustained for a significant depth in the brine pool. As the brine flows toward lower thermal level stages, the condition (Tb)in>T*b>(Tb)out becomes prevalent even for minimum brine depth, which indicates that evaporation may take place only near the surface at the stage inlet and it lessens until it diminishes as the brine approaches the stage outlet, thus making a significant part of the stage nonproductive.
FIG. 5 is a plot of the three losses and the resulting total thermodynamic losses across the stages of a typical prior art MSF unit. FIG. 5 shows the changes in the relative magnitudes of these losses as fractions of the total average temperature difference between flashing and recycling brine along the stages of the MSF unit. Contrary to MSF, non-equilibrium thermodynamic losses are non-existent in the MED process. This is because evaporation occurs in the superheated liquid film rather than by flashing of the liquid pool. On the other hand, both boiling point elevation and pressure drop losses exist to an extent similar to that of the MSF process. However, these losses have far less significance as far as the thermal performance of the MED process is concerned, mainly because the evaporation temperature range is already limited to a narrow low temperature stretch, and also because the overall heat transfer coefficient at these low thermal levels is almost double that of the MSF process.
The combined effect of these losses is illustrated by the per-stage and the accumulated mass flow rates of product distillate shown in FIG. 6 for a typical prior art MSF unit. FIG. 6 clearly shows that stage productivity is directly dependent on stage thermal level, and that low stage productivity in the stages of the lower temperature range in the MSF process is an inherent characteristic.
Two basic quantities must be first established when an MSF or an MED plant is under consideration, namely the plant's production capacity and the available thermal energy in the form of low-grade steam required to drive any of these plants to produce the desired output. The guidelines for measuring MSF and MED plant effectiveness, or the process potential, are usually based on these two quantities and are known in combination as the gain output ratio (GOR) or the performance ratio (PR). The GOR is defined as the mass ratio between the product distillate (in kilograms per unit time) and the steam supplied to the process (also in kilograms per unit time). The PR is defined either as the amount of distillate mass (in kilograms per a predefined quantity of latent heat due to condensation of the heating steam, measured in kilojoules) or the amount of heat supplied (in kilojoules) to produce one kilogram of distillate. These ratios depend on several parameters, some of which are the top brine temperature (TBT), number of evaporation stages or effects, available flashing temperature range, mass ratio of the brine subject to evaporation and the product distillate, concentration of the brine, and effectiveness of evaporation stage or effect. There are, however, certain technical and economic limitations to the upper values of the GOR or PR that can be achieved for any process. However, one must be cautious when comparing these quantities (GOR or PR) for MSF with that of MED, since heating steam conditions, and, hence, the grades of energy supplied to each process, are usually quite different. It would be desirable to be able to integrate MSF with MED such that the flashing temperature range of the MSF process is shifted upward on the temperature scale for better performance of the MSF at relatively higher operating temperatures, while the MED subunit incorporated into the MSF system operates in the lower temperature range for better performance in this range.
Thus, a combination multi-effect distillation and multi-stage flash evaporation system solving the aforementioned problems is desired.